The present invention relates to an ultrasound diagnosis apparatus which can effectively change a width of a transducer array.
In ultrasound diagnosis apparatus, drive pulses generated by a pulse generator are applied to a transducer array. Then, the transducer array is excited by the drive pulses to radiate ultrasound pulses into an object under diagnosis. The echo pulses returned from the object are received by the transducer. The echo acoustic pulses are converted into corresponding electrical signals. The converted electrical signals are properly processed to be visible on a monitor in the form of a tomogram of the object. In ultrasound diagnosis apparatus, the diameter of the ultrasound beam radiated from the transducer array at the portion of the object being studied influences the resolution of the tomogram in the azimuth direction, i.e. a direction orthogonal to a propagating direction of the ultrasound beam, in such a way that the resolution of the tomogram increases as the convergence of the ultrasound beam at the portion under study increases. The diameter of the ultrasound beam is substantially equal to the width of the excited portion in the vicinity of the transducer array. When the transducer is made up of a number of transducer elements, the excited transducer elements may be referred to as the drive portion. In the one piece type transducer, the drive portion corresponds to the entire transducer portion. The resolution of the tomogram in the vincinity of the transducer array can be improved by reducing the width of the drive portion of the transducer array. With transducer arrays, the resolution of the tomogram can be improved by reducing the number of transducer elements. Away from the transducer array, the ultrasound wave expands due to diffusion, so that the diameter of its beam expands as it progresses. The expansion of the beam diameter is inversely proportional to the drive portion of the transducer array. Therefore, if the width of the drive portion is made small for improving the resolution at a location near the transducer, the expansion of the ultrasound beam diameter is large and the resolution at a short range is deteriorated. Thus, for improving the resolution, an apparent contradiction exists in that for improving the resolution in a short range, the width of the drive portion must be small, but for improving the resolution in a long range, it must be large. Further, the expansion of the ultrasound wave is inversely proportional to its frequency. Accordingly, the resolution in the azimuth direction also depends largely on the frequency of the ultrasound. This will be described in detail referring to FIG. 1 illustrating an expanding (ultrasound field) ultrasound beam radiated from a transducer array 1. The transducer array 1 is comprised of a plurality of transducer elements aligned in a series and with the same thickness, as viewed in the azimuth direction. A plan view of these arrayed transducer elements is indicated by reference numeral 1'. As seen from the figure, the diameter of the beam in the vicinity of the transducer array 1 is substantially equal to the width of the transducer array 1, irrespective of the frequency of the ultrasound wave. In a distant region from the transducer array, the beam changes its diameter with the frequency. In the figure, frequencies f1 to f3 of the ultrasound beams increase with their suffix numbers, f1&lt;f2&lt;f3. As seen from the figure, the larger the expansion of the beam, the lower the frequency of the ultrasound beam. Therefore, to make the expansion of the beam diameter small, it is necessary to select the frequency of the beam to be high. In this case, however, another problem arises in that the attenuation of the ultrasound wave in a living body increases with frequency. This implies that the high frequency components of the ultrasound beam become more attenuated as the beam goes deeper into the living body. Accordingly, when imaging deep in the living body, the low frequency components have great contribution to the resolution. In this respect, the frequency of the ultrasound beam must be limited below a given frequency. The consequence is a compromise between a proper beam diameter and a proper beam frequency. More explicitly, at an imaging plane X.sub.l1 distanced "l1" from the transducer 1, the frequency component f3 is attenuated to almost zero. Accordingly, the resolution at this portion is determined by the frequency component at f2 with the beam diameter D2. Accordingly, the resolution at this measuring location depends on a compromise between the frequency f2 and the beam diameter D2. Further, at an imaging plane X.sub.l2 distanced "l2" from the transducer array 1, the frequency components f2 and f3 are both lost and only the frequency component f1 is effective. The resolution at the imaging plane X.sub.l2 is determined by the frequency component f1 and its diameter D3.
For the above background reason, the improvement of the resolution of a tomogram has a limit in both short and long ranges. Particularly, in the long range or in a deep measuring portion of the living body, a high resolution can not be obtained and a picture quality of a tomogram is poor.